1. Field of the Invention
This invention relates to the production of sodium carbonate (soda ash) from trona, and particularly to an improved process for crystallizing sodium carbonate in multipleeffect crystallizers.
2. Description of the Prior Art
Trona, as mined in the area of Green River, Wyoming, contains about 90 to 95 percent sodium sesquicarbonate (Na.sub.2 CO.sub.3 .multidot.NaHCO.sub.3 .multidot.2H.sub.2 O). The remaining 5 to 10 percent consists mainly of insoluble components (principally shale) plus sodium chloride (NaCl), sodium sulfate (Na.sub.2 SO.sub.4), and small quantities of organic matter.
In a common method of obtaining marketable soda ash from this trona ore, the crude trona is first heated to approximately 150.degree.-200.degree. C. to decompose the bicarbonate portion of the sesquicarbonate into sodium carbonate and carbon dioxide. The resulting crude soda ash is mixed with enough water in a dissolver to produce a substantially saturated solution having about 30 percent Na.sub.2 CO.sub.3 by weight. The solution is separated from the insoluble impurities, treated with activated carbon to remove a majority of the organic matter, and then usually filtered to yield a purified clear pregnant liquor, yet which still contains some organic matter plus dissolved NaCl and Na.sub.2 SO.sub.4.
The pregnant liquor is then fed into multiple effect evaporative crystallizers (usually three-stage) in which water is evaporated and crystals of sodium carbonate monohydrate (Na.sub.2 CO.sub.3 .multidot.H.sub.2 O) are formed. The crystals are separated from the mother liquor, dried in a product drier, screened for dust removal, and then stored for shipment as final product. Because the mother liquor may contain as much as 30 percent of the sodium carbonate in the original pregnant liquor feed, it is normally recycled to the crystallizers for further treatment.
The basic process outlined above, also known as the monohydrate process, is disclosed in U.S. Pat No. 2,962,348, issued to L. Seglin et al. on Nov. 29, 1960. U.S. Pat. No. 3,933,977, issued to J. M. Ilardi et al. on Jan. 20, 1976, provides additional detailed description of equipment and procedures for preparing a pregnant liquor carbonate process solution, and the disclosures of both these patents are incorporated here by reference.
Although recycling the mother liquor greatly improves the yield of sodium carbonate crystals, the concentrations of residual organic matter which passes through the carbon treaters and soluble impurities such as NaCl and Na.sub.2 SO.sub.4 tend to build up in the crystallizers. If the organic matter buildup gets too high, it can cause foaming in the crystallizers, and the sodium carbonate crystals will be discolored and have undesirably low bulk density. Excessive concentrations of NaCl and Na.sub.2 SO.sub.4 will result in complex salts which may crystallize out with the sodium carbonate to cause off-specification product. The foaming effect can be offset to some extent by adding defoaming compounds, but the adverse effects on the crystalline product can be prevented only by keeping the concentration of these impurities at a relatively low level.
One solution to this problem is to recycle only a portion of the mother liquor and purge the rest. The previously mentioned Seglin et al. U.S. Pat No. 2,962,348, for example, teaches purging enough mother liquor from the recycling line to operate at a concentration of about 5% combined NaCl and Na.sub.2 SO.sub.4.
In the Seglin et al. U.S. Pat. No. 2,962,348, as well as in subsequent U.S. Pat. Nos. 3,131,996 and 3,655,331 issued to the same inventors, both the fresh pregnant liquor and the recirculated mother liquor pass in series through the entire crystallizer, whether it is single stage or multi-stage, although the U.S. Pat. Nos. 3,131,996 and 3,655,331 suggest alternatively that the mother liquor can be returned to the third stage of a triple-effect evaporator crystallizer, if desired.
U.S. Pat. No. 3,653,848, issued to E. B. Port et al. on Apr. 4, 1972, proposes another method to prevent buildup of impurities, particularly organic matter, in a recycling crystallization process. In the Port et al. process, fresh pregnant liquor is delivered only to the second and third effects of a triple-effect evaporator crystallizer. Mother liquor, separated from a "first crop" of sodium carbonate crystals taken from these crystallizers, is recycled to the highest temperature first effect crystallizer to produce a "second crop" of crystals. A portion of the recycled mother liquor is purged directly from the first effect crystallizer in order to keep the organic impurities level below 5000 ppm, basis sodium carbonate. The remainder of the recycled mother liquor, containing the "second crop" of crystals, is returned to a tank holding a slurry of the "first crop" crystals from the second and third effect crystallizers.
The aforementioned Ilardi et al. U.S. Pat. No. 3,933,977 discloses an alternative process in which a portion of the mother liquor recycled to a monohydrate crystallizer is bled off and directed to a separate anhydrous (higher temperature) crystallizer. A portion of the bled-off mother liquor is purged directly from the anhydrous crystallizer, and an additional amount may be purged after separation from the anhydrous sodium carbonate crystals produced in this latter crystallizer.
Thus, both Ilardi et al. and Port et al. obtain additional sodium carbonate product from at least a portion of the recycled mother liquor before it is purged. The Ilardi et al. process requires a separate high temperature crystallizer. The Port et al. system, on the other hand, uses one stage (the hottest stage) of a triple-effect evaporator crystallizer for segregating all of the recycled mother liquor from the other two stages. This arrangement demands careful control, however, to maintain a balance between fresh pregnant liquor feed and mother liquor recycle rates proportional to the capacities of the respective crystallizer stages.